Unravelling the effects of calcium substitution in BaGd 2 CoO 5 Haldane gap 1D material and its thermoelectric performance

Eco-benign and high-temperature-stable oxides are considered a promising alternative to traditional Bi 2 Te 3 , Bi 2 Se 3 and PbTe-based thermoelectric materials. The quest for high performing thermoelectric oxides is still open and, among other challenges, includes the screening of various materials systems for potentially promising electrical and thermal transport properties. In this work, a new family of acceptor-substituted Haldane gap 1D BaGd 2 CoO 5 dense ceramic materials was characterized in this respect. The substitution of this material with calcium results in a general improvement of the electrical performance, contributed by an interplay between the charge carrier concentration and their mobility. Nevertheless, a relatively low electrical conductivity was measured, reaching ~5 S/cm at 1175 K, resulting in a maximum power factor of ~25 µW/(K · m 2 ) at 1173 K, for BaGd 1.80 Ca 0.20 CoO 5 . On the other hand, the unique anisotropic 1D structure of the prepared materials promotes efficient phonon scattering, leading to low thermal conductivities, rarely observed in oxide electroceramics. While the BaGd 2-x Ca x CoO 5 materials show attractive Seebeck coefficient values in the range 210-440 µV/K, the resulting dimensionless figure of merit is still relatively low, reaching ~0.02 at 1173 K. The substituted BaGd 2-x Ca x CoO 5 ceramics show comparable thermoelectric performance in both inert and air atmospheres. These features highlight the potential relevance of this structure type for thermoelectric application, with future emphasis placed on methods to improve conductivity.


Introduction
In recent years, the rapid depletion of fossil fuel resources and subsequent potential future oil crises, together with environmental-related problems, have led the researchers to focus their work more on alternative energy conversion and storage devices such as batteries, fuel cells, solar cells, supercapacitors and solar thermoelectric generators 1-3 to meet the increased energy demand. Among them, solar thermoelectric generation offers the unique possibility to directly convert solar heat into electrical power, based on the Seebeck effect [4][5][6] . These solid-state devices are robust, reliable and scalable, have no moving parts and do not require maintenance. Despite these advantages, the performance and efficiency of thermoelectric devices is rather low and is limited by several factors, their use being reserved mainly for niche applications, where their advantages outweigh their disadvantages 7,8 . Furthermore, the search for environmentally benign, highly abundant and non-toxic materials is in prime focus for these devices, if targeting a reasonable range of potential applications. To this end, oxide-based thermoelectric materials have been introduced as promising alternatives to the state-ofthe-art 'traditional' critical lead, antimony, bismuth, tellurium and selenium-containing thermoelectric intermetallic alloys, due to their ability to work in the air at high temperatures, without degradation or decomposition 6,8 .
The thermoelectric performance is usually given by the dimensionless figure of merit,

=
, where S is the Seebeck coefficient or thermopower, σ is the electrical conductivity, T is the absolute temperature and κ is the total thermal conductivity 6,[8][9][10] .
High Seebeck coefficient and electrical conductivity, simultaneously with low thermal conductivity, are essential requirements in achieving high ZT values at any given temperature. However, all these parameters are not independent of each-other, meaning that enhancing the charge carrier concentration, for example, a strategy usually pursued in this type of functional materials, must not be performed at the expense of increasing the thermal conductivity [10][11][12]  The cobaltite-based materials were identified to be highly stable at high temperatures, eco-friendly and widely available in nature [16][17][18]29 . The stability of BaR2CoO5 (R = rare earth) materials has been explained based on the global instability index (GII) using a bond valence method, and it was found that the rare-earth-based oxides possess a very high internal stress value of 0.2 v.u. 30 The main structural feature of these materials is the existence of a one-dimensional arrangement of vertex-sharing (CoO6) flattened octahedra, successively parallel to the crystallographic a-axis 31 . These octahedral chains are intertwined with monocapped (RO7) trigonal prisms, while the barium cations are closely surrounded by ten oxygen atoms, forming bicapped (BaO10) square prisms. The very short distance between Co-O-Co atoms is shown to have a profound effect on the behavior of Co 2+ sublattice and is responsible for the typical 1D antiferromagnetic behavior of these materials 30,31 .
The current work assesses the impact of acceptor doping in BaGd2CoO5 Haldane gap material on the structural, microstructural, electrical and thermal properties. The thermoelectric performance of novel BaGd2-xCaxCoO5 (x=0.00-0.30) materials is evaluated in the temperature range 400−1173 K, under air and argon atmospheres.

Experimental
All BaGd2-xCaxCoO5 (x=0.00-0. 30 respectively, for clarity. The phase analysis was performed by powder X-ray diffraction (XRD) using a Philips The thermal diffusivity (D) and specific heat capacity (Cp) studies were conducted with the standard Netzsch LFA 457 Microflash and Netzsch DSC 404F1 instruments, in both flowing air and argon atmospheres, using a measurement procedure identical to that used for the electrical measurements. The total thermal conductivity (κ) was calculated as κ=D×ρ×Cp. The estimated error of the obtained thermal conductivity values was less than 10% 35 .The lattice thermal conductivity (κph) was then evaluated from the wellknown Wiedemann-Franz law as: is the Sommerfeld value of the Lorenz number 36 .

Structural characterization and microstructural evolution
As it was mentioned earlier, all BaGd2-xCaxCoO5 (x=0.00-0.30) Haldane gap ceramics were synthesized in an inert atmosphere, since the desired phase is not formed in air due to the oxidation of Co(II) to Co(III) 30 . The XRD patterns for all samples calcined at 1553 K in argon atmosphere are depicted in Fig. 1. All the samples exhibit apparently single-phase composition, without any secondary peaks visible on the XRD patterns. The identified BaGd2CoO5 Haldane gap phase belongs to an orthorhombic system (Sp. gp: Imma) and its refined lattice parameters and lattice volumes are presented in Table 1 and Fig. 2, as a function of Ca content. The SEM studies (Fig. 3) performed on selected fractured samples reveal high-quality ceramic materials having high density and small porosity. The representative SEM micrographs clearly show that all samples present a similar microstructure, represented by tightly-packed irregular grains of various shapes and sizes. The corresponding densities in each case are listed in Table 2.  The BGCC10 composition lies within the solid solution range and shows a homogeneous distribution of the constituent cations, in agreement with the XRD results ( Fig. 1). On the other hand, the chemical mapping results for BGCC20 suggest the presence of local Ca-and Co-rich impurities, which were previously not detected by XRD, probably due to their small amounts. This fact is apparently responsible for the inflection observed on the unit cell volume vs. composition curves (Fig. 2), at x ≥0.15.
The appearance of such impurities also correlates well with the decrease in relative density observed for x=0.20-0.30 (Table 2). This decrease might be also due to the volatilization of calcium at the high sintering temperature used in this work (1553 K).
Although the deviations from a single-phase composition may affect the interpretation of the TGA results, the observed tendencies shown in Fig. 5 are well-in-line with those expected for acceptor-type substitutions in BaGd2−xCaxCoO5 samples, at least for moderate calcium contents. As compared to the CaxBaGd2−xNiO5 materials 12 , the substituted BaGd2−xCaxCoO5 samples from this work present a rather significant fraction of oxidized transition metal cations, with expected positive impact on the electronic transport, as it can be seen in Fig. 5. Based on the previously proposed defect models for Haldane gap nickelates 12,38 , the corresponding defect chemistry reactions in this case can be described using the standard Kröger-Vink notation 39 as: where β represents the contribution provided by the formation of charge carriers Co 3+ This decrease is likely contributed by the predominant charge compensation by oxygen vacancies and, for higher cobalt contents, by the exsolution of phase impurities (Fig. 4). This is also suggested by the higher variation of Co 3+ content with temperature, observed for the compositions with x≥0.10.
In general, these results indicate that the concentration of electronic charge carriers from the prepared compositions is dramatically affected only at relatively low acceptor contents.

Thermoelectric performance
The electrical conductivity of BaGd2−xCaxCoO5 (x=0.00−0.20) samples was evaluated in the temperature range 570-1175 K, under both Ar and air atmospheres (Fig. 6). A typical semiconducting behavior is found for all measured samples, in the whole measured temperature range. 3-10 times enhanced conductivities as compared to the pristine BGCC0 is observed for calcium-substituted samples, as shown in Fig. 6, in agreement with the calculated density values and the predictions based on the changes of charge carrier concentrations, estimated from TGA studies (Fig. 5). Even higher electrical conductivity is observed for the samples measured in air, as compared to the measurements performed in Ar. In this case, oxygen effectively acts as an electronic acceptor, filling the oxygen vacancies and increasing the concentration of Co 3+ species, combined with similar effects provided by the calcium substitution. This can be described by the following defects reaction: It is worth noticing that under air, the pristine BGCC0 samples show ~3 times higher conductivity as compared to the measurements in Ar. However, the difference in conductivity values measured in Ar and air drastically decreases for Ca-substituted  (Table 3). The variation of the Seebeck coefficient values with temperature is presented in Fig. 7, also under air and Ar atmospheres.  obviously related to the massive charge carrier generation on oxidation, as revealed by the corresponding ~3 times conductivity increase (Fig. 6). While the amount of these charge carriers is enough to suppress the Seebeck coefficient to one of the lowest measured values, the conductivity of BGCC0 in air is still below the rest of the samples, suggesting the lowest mobility. This hypothesis also agrees with the increase in charge carrier mobility, observed for BaGd2−xCaxNiO5 materials, upon substitution with calcium 12 .

Open symbol -air Filled symbol -Ar
The power factor is improved by increasing the temperature and Ca substitution. The BGCC20 sample shows the highest power factor value of ~25 µW/K.m 2 at 1173 K, due to the highest electrical conductivity values from the whole range of studied compositions. It should be noticed that these power factor values are notably below those found/measured for state-of-the-art Ca3Co4O9 and SrTiO3 thermoelectric oxides.
The highest power factor value measured for the BaGd2NiO5 Haldane gap material is ~21 µW/K.m 2 at 1179 K, as previously reported by our group 12 . Thus, in this context, only a marginal enhancement in power factor values is achieved for calcium substituted BaGd2CoO5 compositions.  In the studied materials, the heat is predominantly transported by phonons, and the contribution of the electronic counterpart does not exceed 1%. Corresponding data for lattice thermal conductivity measurements are shown in Fig. 9(b), where the tendencies observed are essentially similar to those found for the total thermal conductivity ( Fig.   9(a)). Therefore, the impact of possible oxygen vacancies on the phonon scattering is apparently negligible, as compared to more pronounced effects observed for strontium titanate-based thermoelectrics 22,24,26,40,41 . This fact most probably stems from the highly anisotropic crystal structure of the Haldane gap materials, which provides rather efficient scattering of phonons, compared to oxygen vacancies, and possibly, to some other type of point defects. The difference between the low thermal conductivity values observed for BaGd2−xCaxCoO5 materials and the typical ones found for donorsubstituted strontium titanates, also justify this assumption. The thermal conductivity values exhibit an increase from pristine BGCC0 to BGCC5; the reasons for this Further promotion of microscale-size ( Fig. 4) phase impurities shifts the chemical composition of the main phase and decreases the advantage of its anisotropy, resulting in the higher thermal conductivity values measured for the BGCC20 samples.
Nevertheless, the thermal conductivity values from this work are among the lowest found in oxides, as illustrated in Table 4. Finally, the temperature dependence of the dimensionless figure-of-merit for all samples is plotted in Fig. 10, between around 450 and 1200 K. The ZT values increase with temperature and calcium substitution, reaching the maximum value of ~0.02 for both BGCC15 and BGCC20 at 1173 K. Despite showing the lowest thermal conductivity values among selected oxide-based thermoelectric materials, the thermoelectric efficiency of the materials studied in this work is still below that acceptable for most thermoelectric applications. The underlying reason for this is mainly low electrical conductivity, which can be possibly improved by exploring further possibilities to design acceptor-(co)substituted Haldane gap oxides.

Conclusions
In this work, the structural, microstructural, electrical and thermal properties of calciumsubstituted BaGd2CoO5 Haldane gap family materials were studied, to assess these